12- The synchronized Keyword

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So you learn how to use locks to implement synchronization.

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Now in Java we have another tool for implementing synchronization and

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that's the synchronized keyword.

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Using the synchronized keyword we can achieve the same result but without explicitly

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creating a lock object and locking and unlocking it.

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So our code looks simpler.

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Let me show you.

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First I'm going to delete the code that we wrote in the last video.

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So delete and delete.

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We also don't need this lock object

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anymore.

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So if we run our program,

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now we should have a race condition.

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There you go.

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So to prevent this,

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we want to make sure that only one thread at a time can execute this piece of

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code. To do that,

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we wrap this inside a synchronized block

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like this.

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Now here in parenthesis,

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we should pass an object.

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This is what we call a monitor object.

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Every object in Java has a built -in lock.

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So Java is going to get the built -in lock from that object and use it under

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the hood.

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For example,

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we can pass this as a reference to the current object,

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but this is a bad practice.

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I will explain why in a second.

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First,

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let's run our program again and see if the synchronized keyword actually solved

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our problem or not.

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So,

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run.

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There you go.

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The problem is fixed.

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But why is passing this a bad practice here?

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well let's add another field in this class private

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integer total files with this we can keep track of how many

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files we have successfully downloaded so far now we need a getter

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for this field so let's create it real quick create getter

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for total files you also need an increment

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method I'm going to type that manually so

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public void,

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increment total bytes,

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sorry, total files,

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and here we increment total files.

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Now,

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we want to make this new method thread safe as well,

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because it is possible that two download threads finish concurrently,

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so we don't want them to update the total files field at the same time.

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So,

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we wrap this inside a synchronized block with this as the

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monitor object,

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like this.

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Now, with this implementation,

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if one thread is calling the increment total bytes method,

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another thread cannot call the increment total bytes method

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because both these methods are using the same monitor objects.

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So only one thread at a time can call into a synchronous method

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of this object.

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This is like having a single hotel room.

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Each monitor object represents a hotel room.

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So with this implementation,

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if one thread is updating the total bytes,

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another thread cannot update the total files field.

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That's going to cause a lot of overhead.

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The whole point of synchronization is to make sure that two threads cannot

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modify the same data at the same time.

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But here we're dealing with two different fields.

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So using the same monitor object reduces the throughput of this

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class.

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It can cause unnecessary weights.

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Now, in a simple application with a few threads,

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you may be able to get away with it,

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but in a larger application with more concurrency requirements,

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you should use dedicated monitor objects.

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Let me show you.

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So,

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we add two new fields to this class.

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Private object total bytes

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lock.

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We set it to a new object.

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And one more time,

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we create another field called total files lock.

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Note that these are just plain object instances.

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They're not lock objects,

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okay? Now,

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by object,

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it's just a common convention.

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Technically,

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we could use any type here,

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but it's more conventional to use the object class because we're not looking for any specific

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behaviors,

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okay?

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Now,

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in this method,

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instead of synchronizing on this,

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we're going to synchronize on total bytes lock

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and

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In this other method,

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we're going to synchronize on total files lock.

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So two different monitor objects.

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Now there's another way to use the synchronize keyword.

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Instead of using it as a block,

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we can declare this entire method as synchronized.

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So we type public synchronized void,

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and this is exactly equivalent to writing synchronized this.

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So this will wrap this entire method inside the synchronized

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block.

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Again, you should be cautious about this approach because this is going to synchronize on the

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current object.

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And as I told you earlier,

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this can cause a lot of unnecessary weights.

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So it's better to use the synchronized block with a dedicated monitor

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object.

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Now,

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all that aside,

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synchronization is bad.

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Your code runs sequentially and you lose concurrency.

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Also,

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you may cause deadlocks and all sorts of crazy bugs that are hard to reproduce and

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fix.

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So as a best practice,

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don't use it in new code.

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I just included in this course in case you're maintaining an older code base

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that uses synchronization.

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So you understand what it is and how it works.

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Next,

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we're going to look at another solution as a lightweight and simpler alternative to synchronization.

12- The synchronized Keyword

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